1. Field of the Invention:
This invention is in the field of chemical ion exchange treatment of a silicate glass to develop strength by compressively stressing a surface layer of the glass. Potassium ions are introduced into said surface layer in exchange for sodium ions.
2. Description of the Prior Art:
It is known to strengthen a glass article containing sodium ions by contact with a molten salt containing alkali metal ions having a diameter greater than that of the sodium ions in the glass. Where potassium ions are substituted for the sodium ions, a compressive stress is developed in the surface layer of the glass article as disclosed in the Journal of the American Ceramic Society, Vol. 45, No. 2 (February 1962) pages 59-68. In the process described in the above article, ion exchange was conducted at a temperature below the strain point of the glass so as to inhibit molecular re-arrangement and viscous flow during ion exchange of the monovalent metal ions migrating into the glass surface. The larger ions from the molten salt in effect are squeezed into sites originally occupied by the smaller alkali metal ions. The compressive stress set up by this crowding effect substantially increases the impact strength of the glass.
In an article entitled "Strengthening by Ion Exchange" in the Journal of the American Ceramic Society, May 1964, pages 215-219, glasses are described which contain substantial amounts of aluminum oxide or zirconium oxide. These glasses are said to be uniquely capable of having strength imparted thereto by an ion exchange process conducted below the strain point of the glass. Such glasses also maintain high strength subsequent to being abraded to simulate ordinary usage.
Ion exchange treatment of alkali metal silicate glasses has been conducted at temperatures above the strain point of the glass as well as below the strain point of the glass. In this process, the smaller lithium ion replaces the larger sodium and/or potassium ions in contrast to the process described above where a larger ion (potassium) replaces a smaller ion (sodium) below the strain point of the glass. In one method of chemical strengthening described in U.S. Pat. No. 2,779,136, a silicate glass containing exchangeable potassium or sodium ions is treated at a temperature above its strain point with a source of lithium ions, for example, a molten lithium salt. The lithium ions migrate into the glass in exchange for potassium or sodium ions which migrate out into the lithium salt. During the exchange process, molecular re-arrangement occurs in the glass since exchange takes place at a temperature above the strain point of the glass. The smaller lithium ions form a new surface layer on the glass having a lower coefficient of expansion than the original glass. As the article cools, compressive stresses are set up by differential thermal expansion.
In copending application Ser. No. 390,742 filed Aug. 23, 1973, now abandoned and assigned to the Assignee of the instant invention, a process is disclosed for treating an alkali metal silicate ophthalmic glass by an ion exchange process utilizing a molten bath of potassium nitrate at a temperature ranging from 760.degree. F to 960.degree. F. This latter temperature is above the strain point of this ophthalmic glass but is well below the softening point of the glass. The process has the advantage that a shorter exchange period is thereby made feasible.
Comparison results obtained in high temperature and low temperature ion exchange processes indicate that the low temperature ion exchange process, that is, one conducted at a temperature below the strain point of the glass results in a glass having a stressed surface layer which is normally relatively shallow and that in order to obtain deeper penetration longer treatment times are required. In the high temperature ion exchange process, that is, one using temperatures above the strain point of the glass, a stressed layer is obtained which is normally relatively deep in comparison to stressed layers obtained by the low temperature ion exchange process. Presumably because molecular re-arrangement can take place, lower compressive stresses are obtained in the stressed layer of the glass treated using this process.
Because a strengthened ophthalmic lens to be capable of providing satisfactory service must not only resist breakage by impact when the lens is newly produced, but also, as a practical matter, must provide resistance to impact even after the lens surfaces have been abraded as will occur from handling and cleaning both in production and by the user, it has been found that the depth of penetration is of at least equal importance in comparison to the desired improvement in compressive stress.
In U.S. Pat. No. 3,790,260, there is provided recognition of the importance of depth of penetration of the compressively stressed surface layer (neutral zone depth) as a means of insuring a satisfactory ophthalmic lens which will resist lens breakage even after abrasion as a result of normal use. The high strength ophthalmic lens disclosed in U.S. Pat. No. 3,790,260 is obtained by limiting the lime content of the ion-exchange glass composition since it has been found that calcium oxide in the glass has a deleterious effect upon the strength after abrading as well as results in a reduction in the depth of the compressively stressed surface layer and, therefore, the patent teaches inclusion of only very minor amounts of lime up to about 3 percent can be tolerated without destroying the desired strength of the lens.
Typically, the ophthalmic glass industry has employed soda-lime-silica base glasses for the production of ophthalmic lenses in which 8-15 percent lime (calcium oxide) is included. The lime-type glass has usually been preferred because of its good chemical durability, the ready availability of high purity raw materials at relatively low cost and because calcium oxide is needed to maintain good melting, forming and processing properties in the glass.